Screw threaded joint for continous-profile tubes

ABSTRACT

A flush-type integral threaded joint with truncated-cone threads, with two scaling surfaces  11′, 12′, 11″, 12 ″) set at the end of the threaded portions ( 7, 8 ), and with two annular scaling shoulders ( 5′, 6′, 5″, 6 ″). The two threaded portions ( 7, 8 ) of the male element ( 1 ) and female element ( 2 ), respectively, have the same value of conicity, and the two sealing surfaces ( 11′, 12′, 11″, 12 ″) are, in one case, conical or cylindrical, and in the other spherical. The threaded portion of each male and female element may have an end area with a thread that is not perfect.

FIELD OF INVENTION

[0001] The present invention relates to an integral threaded joint withzero diametral encumbrance for tubes, in particular for tubes used inthe natural-gas and oil extraction industry. The said tubes may be usedboth as tubing for pumping gas, oil or liquefiable hydrocarbons, and ascasings for the boreholes.

STATE OF THE ART

[0002] In the natural-gas and oil extraction industry, when drilling oilor gas wells, tubings or casings having a pre-defined length are used,which must be joined together at their ends to form strings, in order tobe able to reach the very large depths at which reservoirs of oil, gas,and liquefiable hydrocarbons are located.

[0003] The most commonly used drilling technique is that of drillingwells that start from the surface of the ground or of the sea until theoil field or gas reservoir is reached. The depth of these shafts canattain several thousands of metres. During drilling, the boreholes arelined with metal casings throughout their length. The segments of metalcasing, which are each some ten metres long, are joined together bymeans of threaded joints. These casings then form a tubular stringhaving a constant diameter throughout its length, except at the joints,where the external diameter is generally at least 1 inch (25.4 mm)greater than that of the string itself. In order to line the boreholethroughout its depth, a number of strings are used, which have, forreasons of mechanical resistance and the geological characteristics ofthe formation, decreasing diameters the greater the depth reached by thestring, so as to constitute a telescopic structure. It follows that,since the diameter at the bottom of the shaft is fixed according to thepressure and flow rate of the fluid that is to be extracted, the deeperthe well, the larger its diameter at the surface. A disadvantageousconsequence of this is that the cost of drilling is high, and the amountof material required for these coatings is also high, with consequenthigh costs. A smaller diameter of the well also enables a reduction indrilling times and times for completion of the well. Consequently, it isimportant to reduce the diameter of shafts to the minimum, and hencealso the diameter of the tubes used to make the casings, given the sameamount of fluid to be extracted.

[0004] Once the drilling operations are completed, into the wall of theborehole thus lined there is inserted a tubing, which has the functionof pumping the gas, oil or liquefiable hydrocarbon out of theunderground reservoir. This tubing, which is sunk to the entire depth ofthe well, and which can thus reach lengths of several thousand metres,is also formed by joining tubes each of some ten metres in length bymeans of joints of the type referred to above. Normally, also thistubing has a constant diameter throughout its length, except at thejoints, where the external diameter is generally larger, as in theprevious case.

[0005] In both of the cases referred to above, the tubes are joinedtogether by threaded joints, which may be of the integral type, in whichcase one end of the tube has a male thread and the other end a femalethread, or else of the muffed type, in which case both ends of the tubehave a male thread and are joined together by a threaded sleeve or muffhaving female threads at both ends. Normally, the presence of the jointinvolves an increase in the external diameter of the line at the joint,and this results in an increase in the overall dimensions of the lineand of the bore, which are the greater the greater is the externaldiameter of the joint with respect to the tube.

[0006] In view of the demand from the oil companies to reduce to aminimum the costs for extraction of oil, gas and liquefiablehydrocarbons, considerable efforts have been made to reduce the diameterof the wells, and consequently the diameter of the tubes used.

[0007] In order to limit the external diameter of the tubing, andconsequently the costs for drilling and for the material installed,threaded joints having small diametral dimensions are used. These can bedivided into three types according to the features required and themaximum overall dimensions allowed. A first type, which is frequentlyreferred to as “semi-flush”, is a joint of the muffed type, the externaldiameter of which exceeds the external diameter of the tube by not morethan 6%. A second type, generally referred to as “near-flush”, is ajoint of an integral type, the external diameter of which exceeds thatof the tube by 2-3%. Finally, a third type, referred to as “flush”, isan integral joint, the external diameter of which is equal to theexternal diameter of the tube.

[0008] The choice between the various types of joint is made accordingto the load that the string has to withstand, the pressure that actsinside and/or outside the string, the length of the string, and themaximum diametral dimensions allowable in relation to the diametraldimension of the shaft.

[0009] If the diameter of the joints is reduced, it is necessary to findsolutions for compensating their reduced structural strength. In fact,in the region of the joints the efficiency of the tube is necessarilylower than in the body of the tube since the constructional elements,such as the thread, the seals, and the shoulders, are obtained in thethickness of the wall of the tube, and this in turn leads to a reductionin the net section in critical areas of the male thread and femalethread. Reducing to a minimum the causes of failure of joints is offundamental importance because the failure of the tubes, above all afterthey have been set in operation underground, and hence in a situationwhere it is practically impossible for operators to intervene directlyon the joint in the event of a failure in the latter, may have extremelyserious economic consequences for the extraction plant and may causeconsiderable damage to the environment, particularly in the case wherethe oil or gas reservoir contain aggressive elements.

[0010] For this reason, in the past much effort has been directedtowards improving the joints and bringing them up to an optimal level ofefficiency, endeavouring to achieve a proper balance between the variousrequirements, which are, at times, conflicting, of minimal overalldimensions, maximum structural strength, and tightness to prevent fluidscoming out of or entering the well. The tubes are, in fact, subjected tocompressive, tensile and bending loads and to the pressure produced byfluids acting from outside and/or circulating inside the tubesthemselves.

[0011] The joints must also possess excellent characteristics ofresistance to screwing and seizing.

[0012] The structural problems and the problems of tightness arefrequently aggravated by the temperature of the fluids, their capacityfor causing corrosion, and the environmental conditions existing in theareas of extraction.

[0013] In the past, various solutions have been proposed for jointsaimed at meeting the demands referred to above.

[0014] The U.S. Pat. No. 5,462,315 describes an embodiment of a jointhaving reduced diametral dimensions, which in one variant of theinvention may even be zero. The joint has a central shoulder, whichbears, both on the male side and on the female side, a projection and aslot parallel to the axis of the tube, with homologous surfaces and suchas to mate perfectly with a blocking function for the two members of thejoint. Present on the projections of the shoulder are two sealingsurfaces. The shoulder separates two portions of thread, of a conical orconical-cylindrical shape, radially staggered with respect to oneanother.

[0015] This joint is very efficient, but has a structure that isparticularly complex to make and that involves high production costs.

[0016] The U.S. Pat. No. 5,427,418 describes a joint with zero diametralencumbrance, with a conical thread and a tooth profile with a largeangle of the load flank. Tightness is ensured by the grease trapped inthe thread.

[0017] This joint can achieve high values of efficiency, but does nothave a shoulder designed to protect the joint from possible excessscrewing torque, and hence from excessive stresses that would impair itsfunctionality, and it is not provided with a metal seal.

SUMMARY OF THE INVENTION

[0018] A primary purpose of the present invention is consequently thatof overcoming the drawbacks referred to above, which are presented bythe known joints for tubes, by providing a new flush-type integral jointwhich, albeit having a practically zero diametral encumbrance as otherjoints of the prior art, does not present the disadvantages mentionedpreviously.

[0019] A particular purpose of the present invention is to provide anintegral joint with a diametral dimension not greater than the diametraldimension of the tube throughout its length, which has a reducedproduction cost, at the same time guaranteeing high values of strengthand tightness in situ.

[0020] A further purpose of the present invention is to provide a shapethat facilitates installation.

[0021] The above purposes are achieved by a threaded integral joint fortubes, which, in accordance with claim 1, comprises a male memberprovided, on its outer surface, with a portion of thread having theshape of a truncated cone, and a female element provided on its innersurface with a portion of thread having the shape of a truncated cone,each of said male and female elements being provided with two sealingsurfaces set at opposite axial ends with respect to said respectivethreaded portions, and with functions, in the first case, of externalseal, and in the second case, of internal seal, and with two shouldershaving an annular shape, substantially lying in a plane orthogonal tothe axis of said male and female members, said respective two threadedportions being designed to be screwed together reversibly, one insidethe other, until contact between said two annular shoulders is achieved,said joint being characterized in that said respective two threadedportions of each male and female element have the same value ofconicity, and in that said respective two sealing surfaces of said maleand female elements are one of a conical or cylindrical shape, and theother of a spherical shape.

[0022] Thanks to this embodiment, the joint enables facilitatedinstallation of the tubular string, with reduced risks of seizing of thethread and of the seal, at the same time guaranteeing an optimalstrength and tightness of the string at the joints.

[0023] In a preferred embodiment of the invention, in the internal sealone of the two surfaces has a conical shape, and the other a sphericalshape, whilst in the external seal one of the two surfaces has a conicalor cylindrical shape, and the other has a spherical shape.

[0024] Other preferred embodiments of the invention are described indetail in the dependent claims.

BRIEF DESCRIPTION OF THE FIGURES

[0025] Further characteristics and advantages of the present inventionwill emerge more clearly from the ensuing detailed description of apreferred, but non-exclusive, embodiment of an integral joint of theflush type, illustrated purely by way of non-limiting example with theaid of the attached plate of drawings, in which:

[0026]FIG. 1 represents a sectional view in a plane that passes throughthe longitudinal axis of the joint in conformance with the invention,with the two members in a separate position;

[0027]FIG. 2 represents the joint of FIG. 1 with the members in a joinedposition; and

[0028]FIG. 3 represents an enlargement of a detail of the thread of thejoint illustrated in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0029] With reference to the above figures, the joint according to theinvention comprises two members, or segments of tube, namely, the malepart 1 and the female part 2. The joint defines an internal part 20, inwhich the fluid flows, for example natural gas or oil or other similarfluid, and an external part 30, which may also be filled with gases orliquids of various nature, which are also generally under pressure. Theexternal diameter 3 of dimension D of the tube 2 in the region of thejoint is equal to the external diameter of the tube itself in the partdistant from the joint, minus the tolerances of fabrication of the tube.Also the tube 1, which has a male thread, has an external diameter 4 ofa constant dimension D throughout its length, except for the threadedregion itself.

[0030] The female member 2 of the joint has an internal thread 8 with aconical generatrix. The thread has a conicity with values of between6.25% and 12.5%. The range indicated above is optimal because, on theone hand, the choice of lower values would entail making threads thatare excessively long, with the consequence that it becomes difficult toget the male part to enter the female part, and, on the other hand, thechoice of higher values would mean that too few teeth are available inthe threaded portion, and hence the thread has an insufficient bearingcapacity. In an advantageous variant of the invention, the thread may beperfect throughout its length. At the end of the thread 8 in the insideof the tube 2, this has an annular shoulder 6″ set in a plane orthogonalto the axis of the tube. The female member 2, in the region ofconnection between the shoulder 6″ and the threaded portion 8, has anannular region 11″ with a conical surface. The conicity of this surfaceis between 12.5% and 25% in order to guarantee a good seal with thereciprocal contact surface of the male element 1. The range of valuesproposed proves optimal in relation to the value of the conicity adoptedfor the thread and such as to limit the negative effect of the tensileloads on the effectiveness of the aforesaid seal.

[0031] At the external end of the thread 8 the tube 2 is provided with aspherical surface 12″, which, after screwing with the male member 1,comes into contact with the conical region 12′ of the latter.

[0032] With particular reference to FIG. 3, the profile of the tooth ofthe thread is of the “hooked” type, with the load flank 9 at a negativeangle • with values of between 0° and −10° and with the lead-in flank 10with a positive angle • of between 20° and 45°. These ranges of valuesafford considerable advantages, whilst, at the same time, maintainingease of fabrication of the joint. A load flank with a negative angleenables an effective fit of the two members of the joint and reduces thepossibility of opening of the joint on account of high tensile loads. Alead-in flank with an angle that is positive but not large enableseffective participation of the thread in the resistance to compressiveloads.

[0033] The male member 1 has, in the area of the outer surface whichfaces the threaded region of the female member, a thread 7 set in aperfectly reciprocal way, with portions shaped in a perfectly analogousmanner.

[0034] The connection region 12′ between the external shoulder 5′ andthe start of the threaded portion 7 has a conical surface with values ofconicity of between 0% and 25%. This surface presses against thespherical surface 12″ of the female member after screwing of the joint,and the dimensions and tolerances are chosen in such a way that themetal-metal contact guarantees a tightness that will prevent any liquidor gas under pressure that may be outside the joint from penetrating.

[0035] The male member 1 has a surface 11′ of a spherical shape at itsend, which, after screwing with the female member 2, comes to pressagainst the conical surface 11″ of the female part. Also in this secondregion, there is produced a metal-metal contact pressure between the twomembers 1 and 2, which creates a seal against the pressure of the fluidpresent inside the tube.

[0036] The choice of the two regions 11′, 11″ and 12′, 12″ at the endsof the joint where the metal seals are produced with facing surfaceshaving a spherical and conical shape or a spherical and cylindricalshape, in accordance with the invention, renders the joint lesssensitive to the pressure loads that act on it, and have proved optimalfor thin tubing. In fact, given the slenderness of the ends on which thesealing surfaces 11′ and 12″ are made, the pressure, respectively insideand outside the tube, acting on the aforesaid ends, is very likely tocause their deflection. Consequently, a spherical sealing surface isable to maintain even so an optimal contact, as compared to a seal ofthe truncated-cone type, which in this case, on account of the rotationimposed by the deflection of the end, fails to maintain the contact overthe entire sealing band.

[0037] At the end of the thread 7 set in the outer part of the tube 1,the tube 1 has an annular shoulder 5′ set in a plane orthogonal to theaxis of the tube. After screwing of the male and female memberstogether, the shoulder 6″ of the female member 2 comes to rest againstthe end face 6′ of the male member 1, and the shoulder 5′ of the malemember 1 comes to rest against the end face 5″ of the female member 2.The fabrication of the joint and the manufacturing tolerances are suchthat the inner shoulder never comes into contact before the outer one,which is intrinsically more robust, so as not to cause excessivescrewing stresses. The double shoulder moreover protects the joint frompossible excessive torsional loads (“overtorque”), which may occur bothon account of faulty manoeuvres when screwing, and, above all, inparticular operations during installation in the well. Such loads couldlead to excessive stresses on the joint and impair their functionality.

[0038] The shape of the teeth of the thread of the male member 1 is thesame as that of the thread of the female member 2, referred to above.Advantageously, the thread has a perfect profile throughout the lengthof the threaded portion.

[0039] In a variant according to the invention, it may be envisaged thatthe end region 7′ of the threaded portion 7 of the male element, in theproximity of the annular sealing surface 12′, has a thread with anon-perfect profile. The corresponding region 8 of the female element onthe side 12″, which is set facing the portion 7′, has a perfect thread.The region 8′ at the opposite end of the threaded portion 8, i.e., theregion in the vicinity of the sealing surface 11″, may also itself havea thread with a non-perfect profile, and the corresponding threadedportion 7 of the male member facing it has a perfect thread.

[0040] From what has been said above, the advantages of the jointaccording to the invention are evident, in that it ensures optimaloperating performance and efficiency.

1. An integral threaded joint for tubes, comprising a male member (1) provided, on its outer surface, with a truncated-cone-shaped threaded portion (7) and a female member (2) provided on its inner surface with a truncated-cone-shaped threaded portion (8), each of said male (1) and female (2) elements being provided with two sealing surfaces (11′, 12′, 11′, 12″) which are set at axial ends opposite to said respective threaded portions (7, 8) and with functions, in the first case, of external seal, and in the second case of internal seal, and with two sealing shoulders (5′, 6′, 5″, 6″) of annular shape, lying substantially in a plane orthogonal to the axis of said male and female members (1, 2), said respective two portions of thread (7, 8) being designed to screw together in a reversible way, one inside the other, until contact is produced between said two annular shoulders (5′, 5″, 6′, 6″), said integral threaded joint being characterized in that said respective two threaded portions (7, 8) of said male (1) and female (2) elements have the same value of conicity, and in that said respective two sealing surfaces (11′, 11′, 12′, 12″) of said male (1) and female (2) elements are one of a conical or cylindrical shape and the other of a spherical shape.
 2. The joint according to claim 1, characterized in that in the internal seal the two surfaces are one conical and the other spherical, and in the external seal the two surfaces are one conical or cylindrical, and the other spherical.
 3. The joint according to claim 2, characterized in that the dimension (D) of the external diameter (3) of said female member (2) at the joint is the same as the dimension (D) of the external diameter (4) of the female member (2) itself throughout the rest of its length.
 4. The joint according to claim 3, characterized in that said respective portions of thread (7, 8) of said male member (1) and said female member (2) each have a conicity of equal degree, with a value of between 6.25% and 12.5%.
 5. The joint according to claim 4, characterized in that said respective conical sealing surfaces (12′, 11″) of said male member (1) and said female member (2) have a conicity of between 12.5% and 25%.
 6. The joint according to claim 5, characterized in that the profile of the tooth of the thread has the load flank (9) with a negative angle (α), and the lead-in flank (10) with a positive angle (D).
 7. The joint according to claim 6, characterized in that said negative angle (a) has a value of between 0° and −10°, and said positive angle (β) has a value of between 20° and 45°.
 8. The joint according to claim 7, characterized in that said threaded portions (7, 8) of both the male member (1) and the female member (2) have a perfect thread throughout their length.
 9. The joint according to claim 7, characterized in that the threaded portion (7, 8) of at least one between the male member (1) and the female member (2) has a region (7′, 8′) with a non-perfect thread at one first end.
 10. The joint according to claim 9, characterized in that the threaded portion (7, 8) of the other of said at least one between the male member (1) and the female member (2) also has a region (7′, 8′) with a non-perfect thread at the end axially opposite to said first end. 